FIG. 1 is a systematic view showing a power supply system for a portable computer according to the prior art. As shown in FIG. 1, a power adapter 100 is configured to convert an input AC voltage into an output DC voltage, in which the output DC voltage is used to power a portable computer 102. The portable computer 102 includes a battery pack 104 and a DC-DC converter 106, in which the DC-DC converter 106 is configured to convert the DC voltage provided by the power adapter 100 into a DC voltage required by the internal components of the portable computer 102. The battery pack 104 is configured to power the portable computer 102 in case that the portable computer 102 is not powered by the power adapter 100. Generally, the battery pack 104 can be constituted by a plurality of serially-connected rechargeable batteries. The battery pack 104 is configured to store a portion of the output energy of the power adapter 100 through a battery charger 108 when the portable computer 102 is powered by the power adapter 100 and release the stored energy to the DC-DC converter 106 through voltage rails 109 when the portable computer 102 is not powered by the power adapter 100, thereby enabling the DC-DC converter 106 to provide the voltage required by the internal components of the portable computer 102. The battery charger 108 is configured to downscale the DC voltage provided by the power adapter 100 into a DC voltage which is tailored to charge the battery pack 104, thereby charging the battery pack 104.
FIG. 2 is a circuit block diagram showing the circuit architecture of the power adapter 100 of FIG. 1. As shown in FIG. 2, the power adapter 100 includes a power converter stage 110 which includes a bridge rectifier 121, a switch 122, a switch controller 123, a transformer 124, and a rectifier/filter circuit 125. The bridge rectifier 121 is configured to receive an input AC voltage Vin and convert the input AC voltage Vin into a full-wave rectified DC voltage. The transformer 124 has a primary winding Np and a secondary winding Ns, in which the primary winding Np is connected in series with the switch 122 and configured to store the energy received from the input AC voltage Vin when the switch 122 is ON and transfer the stored energy to the secondary winding Ns when the switch 122 is OFF. The rectifier/filter circuit 125 is constituted by a rectifying diode Dr11 and a filtering capacitor Cf11, and configured to perform rectification and filtration to the energy received by the secondary winding Ns so as to generate a desired output DC voltage Vo. Besides, the power adapter 100 further includes a voltage-limiting circuit 126 and a current-limiting circuit 127. The voltage-limiting circuit 126 includes a voltage divider which is consisted of resistors R11 and R12 and connected to an output terminal of the power converter stage 110. The voltage divider (R11, R12) is configured to generate a fractional output voltage. The voltage-limiting circuit 126 further includes a voltage-limiting amplifier 132 which compares the fractional output voltage provided by the voltage divider (R11, R12) with a first reference voltage Vref11 and in response thereto generates an error control signal. The error control signal is sent to the switch controller 123 through a photo coupler 128 so that the switch controller 123 can regulate the switching duty cycle of the switch 122 according to the error control signal, thereby limiting the output voltage Vo of the power converter stage 110 at a predetermined level. The current-limiting circuit 127 includes a current-sensing resistor Rs11 which is located on the current return path of the power converter stage 110 and configured to generate a sensed voltage being proportional to the output current of the power converter stage 110 when the output current of the power converter stage 110 flows through the current-sensing resistor Rs11. The sensed voltage is coupled to an inverting input terminal of a transconductance amplifier 133, and the non-inverting input terminal of the transconductance amplifier 133 is coupled to a second reference voltage Vref12. The second reference voltage Vref12 is generated by a voltage divider (R13, R14) which is connected to the first reference voltage Vref11, in which the second reference voltage Vref12 is proportional to a maximum current. The transconductance amplifier 133 compares the sensed voltage provided by the current-sensing resistor Rs11 and the second reference voltage Vref12 to determine if the output current of the power converter stage 110 reaches the maximum current. If it is determined that the output current of the power converter stage 110 reaches the maximum current, the transconductance amplifier 133 issues an over-current detection signal. The over-current detection signal is sent to the switch controller 123 through the photo coupler 128 so that the switch controller 123 can limit the output current of the power converter stage 110 at the maximum current to prevent the over-current problems.
The conventional power adapter 100 for portable computer is generally a constant voltage adapter. Because the output voltage of the power adapter 100 is stationary, the output current of the power adapter 100 can determine the output power. According to the constant voltage characteristic of the power adapter 100, the output current of the power adapter 100 will continue rising as the output voltage of the power adapter 100 is stationary. The rising of the output current of the power adapter 100 implies the increase of the output power of the power adapter 100. If the output current of the power adapter 100 exceeds the maximum current, over-power problems would occur. Under this condition, the power adapter 100 is prone to undergo over-heating problems and the lifetime of the power adapter 100 will be shortened accordingly.
In addition, when the power adapter 100 is supplying power to the portable computer 102 through the DC-DC converter 106, the power adapter 100 will charge the battery pack 104 through the battery charger 108 as well. When the battery pack 104 retains a certain amount of electric energy, the battery pack 104 can be charged with a smaller charging current. The decrease of the charging current for the battery pack 104 implies the decrease of the load current provided by the power adapter 100 to the battery pack 104. However, the output voltage of the power adapter 100 is always stationary as stated above. Under this condition, a considerable power loss will be produced on the battery charger 108 and the DC-DC converter 106, and thus the conversion efficiency of the battery charger 108 and the DC-DC converter 106 is reduced.
Hence, if the output voltage of the power adapter 100 can be regulated in response to the load's requirements for load current regulation and the output power of the power adapter 100 can be limited when the output current of the power adapter 100 reaches a threshold current, the over-heating problems of the power adapter 100 can be addressed and the conversion efficiency of the battery charger 108 and the DC-DC converter 106 can be enhanced. Also, the power loss caused by the battery charger 108 and the DC-DC converter 106 can be reduced.
There is a tendency to develop a power adapter capable of providing the function of output power limiting for limiting the output power of the power adapter at a predetermined level when the output current of the power adapter reaches a threshold current and regulating the output voltage in response to the current control signal sent from the load side.